In the production of polyurethane foams, a polyol is reacted with a polyisocyanate in the presence of a polyurethane catalyst and a blowing agent. It is well established in the art that open-cell foams can be produced using water in the reaction mixture to provide a carbon dioxide blowing agent in situ, whereas closed-cell foams are conventionally produced using a flurocarbon blowing agent. The use of water to provide carbon dioxide blowing in the fabrication of open-cell rigid and/or semi-rigid foams has several disadvantages. First, the water in the system facilitates not only the formation of a carbon dioxide blowing agent, but also urea linkages in the polyurethane foam. The presence of these urea linkages in this urea-modified polyurethane foam generally causes unacceptably high foam friability. Second, water is an expensive method of providing an in situ blowing agent since each mole of water consumes a mole of polyisocyanate to provide the carbon dioxide blowing agent and urea linkages. Polyisocyanate is a costly reactant. Thus, in view of the expense and friability problems associated with this conventional method of fabricating open-cell foams, an alternative to the use of water to generate in situ a carbon dioxide blowing agent would be highly desirable.
There is one publication known to the present inventors which discloses that halogenated hydrocarbons can be used as blowing agents in the production of open-cell polyurethane foams, namely, U.S. Pat. No. 3,674,717. More specifically, the '717 patent discloses the production of open-cell polyurethane foam by reacting a starch-phosphorus-based polyether polyol with an organic polyisocyanate in the presence of a polyurethane-forming catalyst and a blowing (or foaming) agent which can be water, halogenated hydrocarbons, and mixtures thereof. This patent does not disclose or suggest the above-discussed disadvantages associated with blowing agents derived from water and, to the contrary, teaches their use. Moreover, although the phosphate-containing open-cell polyurethane foams made from the phosphate-containing polyols disclosed in the '717 patent are useful for many applications, especially those requiring flame retardancy, the foams do not perform as well as might be desired when used in applications that involve contact with a phosphate-hydrolyzable material such as water. For example, when using open-cell foams fabricated from phosphorus-containing polyols as a filter medium for aqueous liquids or moisture-containing gases, the water in these fluids tends to hydrolyze the phosphate linkages in the backbone of the foam, thereby degrading portions of the foam. Such degradation not only diminishes the filtering capability of the foam, but also deposits phosphate-containing impurities in the filtered fluid, which is clearly counterproductive to the filtration function.
In view of the above, it would be highly desirable to provide an open-cell foam that is not subject to degradation in the presence of phosphate-hydrolyzable media and is essentially urea-free.